This invention concerns noise reduction for air induction systems as for internal combustion engines. A portion of the engine noise is propagated back through the air induction system, and in recent years noise attenuation devices have been included in the air induction systems of automotive engines. Such devices have included passive elements such as expansion chambers and Helmholtz resonator chambers connected to air flow ducting in the induction system.
Active devices involving antinoise generators have also been proposed as described in U.S. Pat. No. 5,446,790, issued on Aug. 29, 1995, for an "Intake Sound Control Apparatus". Copending U.S. Ser. No. 08/565,738, filed on Nov. 30, 1995, for a "System and Method for Reducing Engine Noise" describes a compact and efficient packaging of a loudspeaker within an air induction system duct, the loudspeaker driven by an amplified and phase shifted signal received from a microphone positioned to detect noise in an air flow passage.
However, the intensity of the noise reverberating in a confined space within an air duct induction system is considerable, such that it is difficult to control the sound within practical limitations on the power necessary to drive the loudspeaker.
Accordingly, it is the object of the present invention to provide an active noise attenuation system for air induction ducting and particularly in an automotive engine air induction system which requires less power than systems previously proposed for, and in which a more complete cancellation of the noise is radiating from the ducting accomplished.